Bulk Enthalpy of Melting of Poly (l‐lactic acid) (PLLA) Determined by Fast Scanning Chip Calorimetry

Abstract

The bulk enthalpy of melting of 𝜶-crystals of poly (L-lactic acid) (PLLA) is evaluated by fast scanning calorimetry (FSC), by correlating the melting enthalpy of samples of different crystallinity with the corresponding heat capacity at 90 °C, that is at a temperature higher than the glass transition temperature of the bulk amorphous phase and lower than the melting temperature. Extrapolation of this relationship for crystals formed at 140 °C towards the heat capacity of fully solid PLLA yields a value of 104.5±6 J g −1 when melting occurs at 180-200 °C. The analysis relies on a two-phase structure, that is, absence of a vitrified rigid amorphous fraction (RAF) at the temperature of analysis the solid fraction (90 °C). Formation and vitrification of an RAF are suppressed by avoiding continuation of primary crystallization and secondary crystallization during cooling the system from the crystallization temperature of 140 °C to 90 °C, making use of the high cooling capacity of FSC. Small-angle X-ray scattering (SAXS) confirmed thickening of initially grown lamellae which only is possible if these lamellae are not surrounded by a glassy RAF. Linear crystallinity values obtained by SAXS and calorimetrically determined enthalpy-based crystallinities agree close to each other.

Effect of Micro- and Nano-Lignin on the Thermal, Mechanical, and Antioxidant Properties of Biobased PLA–Lignin Composite Films

“…We must report, for the sake of completeness, that more recently, larger values for Δ H 100% of PLA have been estimated. For example, [ 65 ] reported 107 and 143 J/g for the so-called α - and α′ crystal -forms, respectively; whereas, Androsch and coworkers [ 68 ] reported a value of 104.5 ± 6 J/g for poly(L-lactic acid).…”

Physical Aging and Glass Transition Dynamics of Semicrystalline Poly(lactic acid) for 3D-Printing Applications Investigated by Fast Scanning Calorimetry

“…where T g and T a are the glass transition temperature for the used cooling rate of 0.17 K/s (10 K/min) and the annealing/ aging temperature, respectively, and Δc p is the heat-capacity difference of the fully amorphous sample at T g (0.54 J/(K•g) for T g = 333 K). 43 According to the estimation of eq 9, the maximum relaxation strength, which is indicated in the various plots of Figure 7, for all samples is not reached. It is worth noting that the value of Δh re is normalized according to the MAF.…”

“…74 The minor undershoot comes from the compensation lag of the PID controller, which for the used instrument is not smoothed out (electronically or numerically). 58 The crystalline fraction (χ c ) is estimated by comparing the measured melting enthalpy (Δh m ) with the bulk heat of fusion of the α′-form of PLA at the actual melting temperature of 423 K (Δh m∞ (423 K) = 72 J/g) 43…”

Sustainable Electrospun Poly(l-lactic acid) Fibers for Controlled Release of the Mosquito-Repellent Ethyl Butylacetylaminopropionate (IR3535)

“…In addition, the melting temperature of crystals in PLLA/IR3535 fibers systematically decreases with increasing amount of IR3535, which likely is attributed to the solvent-caused equilibrium melting point depression according to Flory. − Quantitative data about melting-peak temperatures as a function of the repellent content are shown with the squares and left axis in Figure b, while the circles and right axis provide information about the polymer content-normalized total enthalpy change (considering both the enthalpy of melting and enthalpy of cold-crystallization) during heating, being proportional to the enthalpy-based PLLA crystallinity. − As such, the crystallinity in neat PLLA fibers is negligible, while for fibers containing IR3535, sizable amount of crystals is evident. A rough estimation of the crystal fraction in fibers containing 42 m% IR3535 (sample P10IR4) yields a value of 38% [= 34 J/g (measured enthalpy of crystallization)/90 J/g (bulk enthalpy of melting of α-crystals with a melting temperature of 150 °C) , × 100%]. Obviously, the addition of IR3535 increases the mobility of the polymer chains, causing a decrease in the glass transition temperature and allowing for faster crystallization of PLLA.…”

“…50−52 As such, the crystallinity in neat PLLA fibers is negligible, while for fibers containing IR3535, sizable amount of crystals is evident. A rough estimation of the crystal fraction in fibers containing 42 m% IR3535 (sample P10IR4) yields a value of 38% [= 34 J/g (measured enthalpy of crystallization)/90 J/g (bulk enthalpy of melting of α-crystals with a melting temperature of 150 °C) 50,52 × 100%].…”